1. Field of the Invention
The present invention relates to a semiconductor device for absorbing a noise included in an external signal, and more particularly to buffers for receiving an external signal to control active/stand-by states of circuits and for shaping the waveform of the external signals.
2. Description of the Prior Art
Digital semiconductor devices such as memories and logic circuits need to be operated at high speed. To realize high speed operation, output circuits of semiconductor devices improve current-absorbing capacity and current-supply capacity to make it possible to drive a load at high speed.
However, when the load is driven at high speed, its capacitance is charged and discharged at a very high rate, thereby producing a current peak. This current peak acts on the inductive components of various wirings for supplying power to the semiconductor chip, thus causing the source voltage of the chip to vary considerably. This variation may cause the semiconductor devices to fail to function properly. Thus, measures are required to prevent such failures.
In semiconductor devices, active/stand-by control is performed to reduce power consumption. A buffer which receives an external signal to issue an output signal for performing an active/stand-by control may be caused to issue an error output signal by a variation in the chip's source voltage Vss. If it occurs, circuits may erroneously change from active state to stand-by state. Consequently, the buffer must be improved.
As shown in FIG. 1, a conventional semiconductor device or chip is provided with a single buffer for receiving an external signal for performing an active/stand-by control of the chip and for issuing an output signal to the chip's internal circuits and to its output circuit of the chip. That is, the buffer receives the external signal (chip enable signal: CE signal) and performs waveform shaping of the external signal to issue t he output or active/stand-by control signal to the chip's internal circuits and to its output circuit. This buffer 10 is constructed of a plurality of inverter stages, and is very sensitive to the external signal for minimizing the delay in the output of the active/stand-by control signal in response to the external signal. This is because it is an object of the buffer 10 to perform an active/stand-by control of the semiconductor chip. Consequently, if the external signal contains some noise, such noise is immediately issued from the buffer 10, thus causing the semiconductor chip to malfunction. In recent years, the buffer's sensitivity has been further increased along with shortening of a chip access time, thus increasing the tendency of the semiconductor chip to malfunction.
As described above, the need for high speed operation of the semiconductor chip makes the buffer for performing active/stand-by control of the chip more sensitive. Thus, the noise in the external signal tends to make the semiconductor chip malfunction.